1. Field of the Invention
This invention relates to a steam oxidation method of subjecting a matter to be oxidized housed in a reactor to steam oxidation and more particularly to a steam oxidation method of subjecting a matter to be oxidized to steam oxidation with proper controllability and reproducibilty when carrying out steam oxidation.
2. Description of Related Art
The steam oxidation method is frequently used, for example, for forming an oxidation confining type, current-confining layer of a surface emitting laser element.
The surface emitting laser element is a semiconductor laser element emitting laser light in vertical direction relative to a substrate surface. As a surface emitting laser element of an 850 nm wavelength band, attention is being drawn to a surface emitting laser element formed on a semiconductor substrate such as GaAs, comprising a pair of DBRs (Diffractive Bragg Reflector) consisting of a pair of AlGaAs/AlGaAs or the like of mutually different Al composition, and an active layer of an AlGaAs type that will serve as a light-emitting region provided between that pair of DBRs.
In such a surface emitting laser element, to enhance a light-emitting efficiency and lower a threshold current, it is necessary to limit a cross-sectional area of a current path of a current to be injected into the active layer. Conventionally, there is employed a method such as providing higher resistance of an ion injected region through an H+ ion injection. However, in recent years, as a method of limiting this current path, there is a mainstream method of forming a current-confining structure by letting a high Al containing layer such as an AlAs layer interposed in a multi-layer film and oxidizing selectively a predetermined area of the high Al containing layer for conversion thereof to Al2 O3 of high electric resistance.
Referring to FIG. 2, description will be made of an example of a construction of a surface emitting laser element having a current-confining structure formed by oxidation of the AlAs layer. FIG. 2 is a sectional view showing the construction of the surface emitting laser element.
A surface emitting laser element 10 is, as shown in FIG. 2, a multi-layered structure comprising a lower DBR 14 composed of an n-type semiconductor multi-layer film, an Al0.3 Ga0.7As lower clad layer 16, an active layer 18, an Al0.3Ga0.7As upper clad layer 20, an upper DBR 22 composed of a p-type semiconductor multi-layer film, and a p-type GaAs contact layer 24, all of which are sequentially formed on an n-type GaAs substrate 12.
The lower DBR 14 is constructed as a semiconductor multi-layer film with an n-type Al0.2Ga0.8As layer and an n-type Al0.9Ga0.1As layer.
The upper DBR 22 as a semiconductor multi-layer film with a p-type Al0.2Ga0.8As layer and a p-type Al0.9Ga0.1As layer.
Also, the p-type Al0.9Ga0.1As layer of a first pair in the upper DBR 22 adjacent to the upper clad layer 20 is, in lieu of the p-type Al0.9Ga0.1As layer, replaced with a p-type Al As layer 26a, and the AlAs layer, excluding a circular area in the center, is selectively oxidized and converted to an Al oxidized layer 26b. 
Namely, this layer constitutes a current-confining layer 26 where the Al oxidized layer 26b functions as an oxidation confining type, current-confining region of high electric resistance and where the AlAs layer 26a functions as a current injection region.
The contact layer 24 and the upper DBR 22 are subjected to etching and processed to a columnar mesa post 20 of a circular cross section.
The contact layer 24 on an upper surface of the mesa post 30 opens a light-emitting window 32 in a vicinity of the center and is formed in a circular ring shape.
In manufacturing the surface emitting laser element 10, as shown in FIG. 3, a multi-layered structure is formed by depositing first on the n-type GaAs substrate 12, in the order of the lower DBR 14, the lower clad layer 16, the active layer 18, the upper clad layer 20, the upper DBR 22 having the AlAs layer 26a, and the contact layer 24.
Next, the vicinity of the center of the contact layer 24 is removed and the light-emitting window 32 is opened.
Subsequently, the reactive ion beam etching method (RIBE) is used to etch the contact layer 24 and the upper DBR 22 up to the upper clad layer 20, thus forming the columnar mesa post 30.
Consequently, there is obtained a semiconductor substrate 44 consisting of a multi-layered structure having the mesa post 30 as shown in FIG. 3.
Next, the semiconductor substrate 44 is heated in a steam atmosphere to oxidize the AlAs layer 26a until a desired oxidation confining diameter is obtained.
In the AlAs layer 26a on the upper DBR 22, AlAs on the periphery of the mesa post 30 is selectively oxidized, generating the Al oxidized layer 26b, while, at the same time, a central region of the mesa post structure 30 remains as the original AlAs layer 26a. 
In forming a current-confining structure of the oxidation confining type into a semiconductor substrate by subjecting a high Al containing layer such as the AlAs layer 26a to steam oxidation, a steam oxidation apparatus described below will be used. Referring to FIG. 4, an example of a construction of a steam oxidation apparatus for subjecting the high Al containing layer to steam oxidation will be explained. FIG. 4 is a schematic diagram showing the construction of the steam oxidation apparatus 40. The steam oxidation apparatus 40 is an invention disclosed in patent application Ser. No. 2003-14260.
The steam oxidation apparatus 40 is an apparatus to be used when forming a current-confining structure into a surface emitting laser element by subjecting the high Al containing layer to steam oxidation. As shown in FIG. 4, as a reactor carrying out steam oxidation, it is equipped with a horizontal-type reactor 42 of a single-slice treatment type.
The reactor 42 comprises a quartz chamber 48 in a horizontal square tube type, an electric heater 50 set up around the quartz chamber 48, and a susceptor 46 housed in the quartz chamber 48, which supports a semiconductor substrate 44 having a multi-layered structure in which the above-mentioned mesa post 30 is formed.
The electric heater 50 is a lamp heater, being capable of increasing a substrate temperature of the semiconductor substrate 44 by irradiation of the lamp.
Further, the steam oxidation apparatus 40 comprises a steam-accompanied inert gas system supplying a steam-accompanied inert gas to the reactor 42, an inert gas system supplying an inert gas to the reactor 42, a reactor bypass pipe 52 subjecting the steam-accompanied inert gas system and the inert gas system to reactor bypassing, and an exhaust system venting a gas discharged from the reactor 42.
The exhaust system has a water-cooled trap 54, comprising a gas discharge port 42B of the reactor 42 and a 4 th gas pipe 56 which leads a gas transmitted from the reactor bypass pipe 52 to the water-cooled trap 54, and a 5th gas pipe 58 which exhausts a gas that passed through the water-cooled trap 54.
The steam-accompanied inert gas system consists of an H2O bubbler which houses pure water, into which an inert gas is transmitted to cause bubbling, and which generates a steam-accompanied inert gas; a 1st gas pipe 64 which is connected to an inert gas source transmits an inert gas whose flow is controlled by an MFC (Mass Flow Controller) 62A into the H2O bubbler; and a 2nd gas pipe 68 which transmits a steam-accompanied inert gas generated in the H2O bubbler through an automatic valve 66A into a gas flow-in port 42A of the reactor 42.
The inert gas system includes a 3rd gas pipe 70 which is connected to an inert gas source and transmits an inert gas, whose flow is controlled by the MFC 62B, through an automatic valve 66C to a gas flow-in port of the reactor 42.
The reactor bypass pipe 52 has its one end connected to the 2nd gas pipe 68 through an automatic valve 66B, and it is connected to the 3rd gas pipe 70 through an automatic valve 66D, its other end being connected to the 4 th gas pipe 56, whereby the steam-accompanied inert gas and the inert gas are subjected to reactor bypassing.
When supplying the steam-accompanied inert gas supplied from the steam-accompanied inert gas system to the reactor 42, the automatic valve 66A is opened, and an automatic valve 66B is closed. When supplying the inert gas from the inert gas system to the reactor 42, the automatic valve 66C is opened, and the automatic valve 66D is closed.
Also, by closing the automatic valve 66A and opening the automatic valve 66B, it is possible to convey the steam-accompanied inert gas supplied from the steam-accompanied inert gas system to the reactor bypass pipe 52. By closing the automatic valve 66C and opening the automatic valve 66D, it is possible to convey the inert gas supplied from the inert gas system to the reactor bypass pipe 52.
The H2O bubbler 60 is housed in a constant-temperature bath 72, and water in the H2O bubbler 60 is held at a predetermined temperature by the constant-temperature bath 72 and by the inert gas flow which is controlled by the MFC 62A.
Related Art Example
Steam oxidation of a high Al containing layer such as the above-mentioned AlAs layer 26a has thus far been carried out as follows by using steam oxidation equipment 40, one example of which is shown in FIG. 4. Referring to FIG. 5, a steam oxidation method for oxidizing the AlAs layer 26a of the semiconductor substrate 44 will be described. FIG. 5 is a time table showing a sequence of the conventional steam oxidation method.
First, an operating condition of the constant-temperature bath 72 and a flow rate condition of MFC 62A are set such that the temperature of the H2O bubbler 60 is held at 80° C. at all times.
Next, the lamp heater 50 is turned on, and at a time point (a), the semiconductor substrate 44 in the normal temperature (approx. 30° C.), that is, the multi-layered structure in which the above-mentioned mesa post 30 was formed is inserted into the reactor 24. Then, at a time point (b) when the temperature of the semiconductor substrate 44 reaches 450° C., supply of N2 gas is started and continued for 3 minutes.
Subsequently, at a time point (c) of 3 minutes after the supply of N2 gas is started, the supply of the steam-accompanied N2 gas in lieu of N2 gas is started. While supplying the steam-accompanied N2 gas, at a time point (d) after a lapse of a preset time, that is, at a time point when a predetermined region of the AlAs layer 26a of the semiconductor substrate 44 is oxidized in steam, the supply of the steam-accompanied N2 gas is stopped. The semiconductor substrate 44 is cooled to the normal temperature and the semiconductor substrate 44 is taken out from the reactor 42.
As mentioned above, it is possible that the AlAs layer 26a of the semiconductor substrate 44 is oxidized in steam, thus forming the current-confining structure of the oxidation confining type.
Since related art technical documents regarding the conventional steam oxidation method described above were not available, disclosure of the related art technical information is omitted.